The present invention generally relates to leading edge protective strips suitable for composite airfoils. In particular, the present invention relates to metals and alloys employed in protecting the metal leading edge (MLE) of airfoil components.
Many modern gas turbine engine airfoil components, such as fan blades and stator vanes, are constructed of composite laminate or molded fiber. MLEs are used to protect the airfoils of such composite components from impact and erosion damage that can often occur in the engine environment. In conventional practices, a V-shaped protective metallic strip is often wrapped around the leading edge and sides of the airfoil to provide such protection.
Many processes exist for the manufacture of such MLEs and will not be discussed in detail here. Related emerging processes have been disclosed in US Patent applications US2011/0129600A1 and US2011/0097213A1. Additionally, other manufacturing processes, including, for example, traditional machining and creep forming can be used to manufacture MLEs. Creep forming is known in the art as a process in which temperatures high enough to cause plastic deformation of a part are used under pressure to effect the desired shape for the part. An aging treatment can simultaneously occur at the process temperature. To date, a large number of MLEs have been composed primarily of titanium or its alloys due to ease of manufacture and reduced weight. Further, the impact strength of MLEs made of titanium or its alloys has been found to be satisfactory. In US patent application US 2011/0194941A1, Parkin et al. disclose a co-cured sheath for a composite blade. While not intending to promote any particular interpretation, it appears to disclose a method of co-curing the MLE with a polymer matrix composite (PMC) blade structure using a sheath made of titanium, nickel, a titanium alloy or a nickel alloy. Though thinner turbine engine airfoils increase overall blade efficiency, thereby reducing engine specific fuel consumption (SFC), any reduction in the thickness of MLEs made of titanium or its alloys may lead to reduced impact strength of the component.
Impact conditions play a large role in sizing an airfoil. Regardless of the thickness of an airfoil, the MLE must be able to withstand the same or similar impact conditions as does the airfoil. Therefore, in order to significantly reduce the thickness of an engine airfoil, an MLE capable of withstanding the same impact conditions with a lower thickness and hence a lesser volume of material is desired. The utilization of a more dense material with higher yield strength allows for an overall thinner structure resulting in system mechanical properties, such as, for example, bending strength and radial mass, nearly identical to those of titanium or its alloys. One particular goal of aircraft engine research and development is improved SFC, which describes the fuel efficiency of an engine design with respect to, for example, thrust output. Since the metal leading edge is applied to protect the underlying composite blade, a leading edge with reduced thickness without compromising mechanical properties can enhance specific fuel consumption, while at the same time affording the needed protection for the composite blade. Thus, there is an ongoing effort to develop materials capable of improving specific fuel consumption without compromising mechanical properties desired for an MLE.